Rowly,
Only time will tell for sure, but looks like you probably didn't have a major fish kill. Sounds like your 4 acres of water 12 feet deep and the size of your pond is a definte plus. However, just because fish do not float up does not mean there are not some laying on the bottom. I know a owner that lost all of his fish and very few floated up. Not sure why but some fish float up when they die and some do not. Maybe it has to do with the rate of decompostion and internal gases building up -- maybe someone on here knows.
You still have a danger of stressed fish succumbing to bacterial problems once the water warms up.
Hard to say for sure, but I would guesstimate you lucked out, and did not have a major fish kill. One fly in ointment though. I was always under the impression that perch are hardier than bass and bluegills and require less oxygen?
